Transistors formed over a glass substrate or the like are typically manufactured using amorphous silicon, polycrystalline silicon, or the like, as typically seen in liquid crystal display devices. Although transistors including amorphous silicon have low field effect mobility, they can be formed over a larger glass substrate. On the other hand, although transistors including polycrystalline silicon have high field effect mobility, they are not suitable for being formed over a larger glass substrate.
In view of the foregoing, attention has been drawn to a technique by which a transistor is manufactured using an oxide semiconductor, and such a transistor is applied to an electronic appliance or an optical device. For example, a technique of manufacturing a transistor by using zinc oxide or In—Ga—Zn—O-based oxide as oxide semiconductor, and using the transistor for a switching element of a pixel of a display device and the like is disclosed in Patent Document 1 and Patent Document 2.